It has been a mild flu season so far, but influenza activity has gradually started to increase in recent weeks, the CDC reports.

Action Points

The 2011-2012 influenza season apparently did not begin until early February.

Note that, as happened last year, rates of influenza-associated hospitalization this season have been highest in children under age 5 and adults 65 and older.

It has been a mild flu season so far, but influenza activity has gradually started to increase in recent weeks, the CDC is reporting.

The start of the influenza season is usually marked when 10% of respiratory samples taken in cases of influenza-like illness test positive for the virus, according to Joseph Bresee, MD, of the CDC.

This year, that mark wasn't reached until the week ending Feb. 4; the percentage rose to 13.4% in the week ending Feb. 11, and 14.4% in the week ending Feb. 18, Bresee said in a telephone news conference.

It's the "latest start to the flu season in 20 years," he told reporters. "The pattern is unusual, but not unprecedented."

The peak of the flu season usually occurs in February, CDC researchers noted in the Feb. 24 issue of Morbidity and Mortality Weekly Report, but "substantial activity" can occur as late as May.

Indeed, Bresee said, in the past 35 years the peak of the flu season has occurred four times in March and twice in April.

For that reason, the agency is still urging people to get a flu shot.

In a separate report, the CDC found that having a doctor suggest or offer a flu shot markedly increased the rate of vaccination among pregnant women during the pandemic season of 2009-2010.

Data from 29 states and New York City, which had added flu shot questions to regular surveillance of women having live births, showed that the median seasonal vaccination coverage among women with a live birth was 47.1%, ranging from 26.1% in Florida to 67.9% in Minnesota.

The median coverage with the monovalent pandemic H1N1 vaccine was 40.4%, ranging from 21.9% in Mississippi to 63.3% in Vermont.

The median prevalence of a provider offer or recommendation for seasonal vaccination was 70.7% and for the pandemic vaccine was 73.3%.

Vaccination coverage was higher if a women's healthcare provider offered or recommended the shots. Median coverage was 62.1% for seasonal vaccine and 53.1% for pandemic vaccine if there was an offer or recommendation, compared with 14.3% and 4.9%, respectively, if there was not.

The implication, the agency said, is that healthcare providers play a key role in getting people to accept vaccination.

The trivalent vaccine for this season includes the pandemic H1N1 strain, an influenza H3N2 strain, and a B virus, the CDC noted. So far, testing shows that most circulating strains match the vaccine well, with the H3N2 strain predominating, the agency reported.

Since October 2, outpatient visits for influenza-like illness (ILI), reported by the Outpatient ILI Surveillance Network, have been between 1.1% and 2.1% of all visits, below the national baseline of 2.4%.

The cumulative incidence from October 2 to February 11 of hospital admissions for lab-confirmed flu was one per 100,000 for all age groups, but was higher for those under age 5 and 65 or older -- 2.2 and 3.2 per 100,000, respectively.

In contrast, the end of season cumulative incidence for admissions in 2010-2011 was 21.6 per 100,000, the agency noted.

For the week ending February 11, pneumonia and influenza was either an underlying or contributing cause of death for 6.7% of all deaths reported to the CDC's 122 Cities Mortality Reporting System.

Since October 2, the weekly percentage of deaths attributed to pneumonia has varied from 5.9% to 7.9%, but has not exceeded the epidemic threshold of 7.9% for more than a week, the CDC reported.

The agency also found that, as of February 11, there have been just three children whose deaths have been linked with influenza. Last season, in contrast, there were a total of 122 influenza-related pediatric deaths.

Tha analyses were conducted by the CDC. Authors are employed by the agency.

Reviewed by Zalman S. Agus, MD Emeritus Professor, Perelman School of Medicine at the University of Pennsylvania and Dorothy Caputo, MA, RN, BC-ADM, CDE, Nurse Planner

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